The present invention provides a III-nitride compound semiconductor device, which is characterized by having a II-nitride compound crystal-island layer in the structure to reduce dislocation of the III-nitride compound occurred during epitaxial growth.
III-nitride compound semiconductors, especially semiconductors with GaN-based material are frequently applied to produce light emitting devices such as blue-green light emitting diodes (LED) and laser diodes. These materials usually grow on aluminum oxide (Al2O3) substrates or silicon carbide (SiC) substrates.
Take the aluminum oxide substrates for example. Because the difference between the lattice constants of Al2O3 substrates and GaN exceeds 16%, a GaN crystal layer is hard to directly grow on an Al2O3 substrate. In U.S. Pat. No. 4,855,249, therefore, Akasaki et al. first disclosed to grow an amorphous AlN buffer layer on an Al2O3 substrate at a low temperature so as to reduce problem caused by the lattice constant difference between an Al2O3 substrate and a GaN layer. Nakamura et al., in U.S. Pat. No. 5,290,393, disclosed to use materials such as GaN or AlGaN to grow as a buffer layer. An amorphous GaN buffer layer was first growing on an Al2O3 substrate at a temperature between 400xc2x0 C. and 900xc2x0 C. A GaN epitaxy layer was then growing on the GaN buffer layer at a temperature between 1000xc2x0 C. and 1200xc2x0 C. The quality and performance of the GaN epitaxy layer were better than those of a GaN epitaxy layer produced by adopting AlN as a buffer layer.
However, because of dislocation defects caused by the difference between the lattice contants of Al2O3 substrates and GaN materials, even GaN-based, AlGaN-based or AlN-based materials are provided as buffer layer materials, epitaxy layer with GaN-based material still has a dislocation density of 1010 cmxe2x88x922 to 108 cmxe2x88x922. This leads to a bad performance of the semiconductor device and affects illumination and electrical property. Thus, methods on how to reduce dislocation density, such as multiple buffer layer structure, epitaxy lateral overgrowth (ELOG) structure, InGaN/GaN superlattice structure, or AlGaN/GaN suprelattice structure, are brought up one after another to reduce dislocation.
The present invention provides a II-nitride compound material to grow directly on the substrate. This II-nitride compound material grows evenly on the substrate or on the III-nitride compound material with crystal-island structure. The III-nitride compound semiconductor layer is to grow thereon to reduce dislocation of the III-nitride semiconductor layer and improve the epitaxy quality.
The present invention discloses a light emitting device with a single crystal island structure. The materials of the single crystal-island layer are II-nitride compounds, wherein the II-group elements include beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), zinc (Zn), cadmium (Cd), and mercury (Hg) etc. The single crystal-island layer grows on the substrate with a certain distance between each two of the single crystal islands. III-nitride compound semiconductor layer then grows on the single crystal-island layer. Because III-nitride compound semiconductor layer grows along II-nitride compound single crystal islands, dislocation occurred during epitaxy is to be confined to where II-nitride compound single crystal island is. Hence, dislocation density is to be reduced effectively.